1. Field of the Invention
The present invention relates to a method of manufacturing a thin and lightweight semiconductor device. Specifically, the present invention relates to a method for manufacturing a semiconductor device over an organic resin member or a plastic substrate. In the present invention, a semiconductor device is a semiconductor device including a semiconductor element having an amorphous semiconductor film as an active region and a semiconductor element having a crystalline semiconductor film as an active region, specifically, a semiconductor device including an optical sensor element, a photoelectric conversion device, a solar battery, or the like.
2. Description of Related Art
An optical sensor is used as a sensor for converting an image into an electrical signal in a wide range of fields such as a facsimile machine, a copying machine, a video camera, and a digital still camera. Mainly, a semiconductor is used as a material of the optical sensor, and silicon can be given as a typical example of a semiconductor material. As the optical sensor using silicon, there are an optical sensor using a single crystal silicon or polysilicon film and an optical sensor using an amorphous silicon film. The optical sensor using the single crystal silicon or polysilicon film has a sensitivity peak in an infrared region in the proximity of 800 nm, and has sensitivity up to in the proximity of 1100 nm. Consequently, in the case of sensing a white fluorescent light which hardly includes a spectrum of an infrared region and sunlight which has a wide range of spectra from an ultraviolet region to an infrared region, there is a problem that detected results of each light are different even though actual illuminance is equal.
On the other hand, the optical sensor using amorphous silicon has little sensitivity to light in an infrared region, a sensitivity peak in the proximity of from 500 nm to 600 nm which is in the center of wavelength of a visible light region, and sensing characteristics similar to human spectral luminous efficacy. Therefore, the one using amorphous silicon is preferable as the optical sensor.
The optical sensor using amorphous silicon can be roughly divided into 1) a resistor type and 2) a diode type. The resistor type can provide a large current since it has an amplification effect as a transistor. However, an amplified optical charge does not disappear even after light is blocked since a large quantity of optical charges is generated by amplification. Therefore, response speed thereof is slow, and a dynamic range due to contrast of light is narrow.
Meanwhile, the diode type optical sensor has a depletion layer spreading in amorphous silicon, and easily detects an optical charge generated when light enters. It has high response speed since it does not have an amplification effect, and a dynamic range due to contrast of light is wide. However, a capacitor for retaining a charge or an element for amplifying and outputting an optical charge is required since current due to the optical charge is small.
As an element for amplifying and outputting a current detected by the optical sensor as an output signal by time division (hereinafter, referred to as an amplifying element), there are a bare IC type using a field effect transistor of a single crystal semiconductor (mainly, a silicon semiconductor) and a TFT type using a thin film transistor using a thin amorphous silicon film or polysilicon film as a channel formation region.
An IC type optical sensor has high speed and high reliability as the amplifying element. However, cost is very high since as many bare chip ICs as the optical sensors are required. In addition, both a substrate over which a photoelectric conversion element (photoelectric conversion layer) made of amorphous silicon or the like is to be formed and a bare IC chip are necessary. Therefore, occupied area in an installation substrate such as a printed wiring board is enlarged, which becomes an obstacle to downsizing of an electronic device equipped with the optical sensor.
Meanwhile, an active region of a TFT that is the amplifying element and a photoelectric conversion layer of the photoelectric conversion element can be formed over the same substrate in the TFT type optical sensor. Therefore, occupied area in an installation substrate such as a printed wiring board can be narrowed; consequently, downsizing of the electronic device equipped with the optical sensor is easy. Further, a cost thereof is low compared with that of the IC type optical sensor using single crystal silicon. Since a TFT using a polysilicon film has higher electrical characteristics than that of a TFT using an amorphous silicon film, high-speed response as the amplifying element is possible. Accordingly, it is effective for detecting even a weak photoelectric current to form the amplifying element with the TFT using a polysilicon film (for example, Japanese Patent Laid-Open No. H6-275808 (pp. 3 to 4, FIG. 1)).
However, the optical sensor using a TFT having an active region made of a polysilicon film as the amplifying element, as disclosed in Japanese Patent Laid-Open No. H6-275808, has limitation on the kind of a substrate due to its manufacturing process. Typically, only a substrate that can withstand crystallization temperature or activation temperature of silicon, such as quartz or glass can be used. This is because a heating step at a comparatively high temperature (for example, equal to or more than 500° C.) is required for crystallization or activation of silicon. These substrates have thick film thickness, which causes a problem of increase in parts volume and weight of the optical sensor.
Since glass or the like does not have flexibility, an installation site of the optical sensor is over a flat portion, specifically, over a printed wiring board, and there is limitation on the installation site. Therefore, an attempt to use lightweight and thin, preferably, flexible plastic, or the like as a substrate of the optical sensor has been made. Plastic is lightweight and thin; however, it has a low allowable temperature limit. There is a problem that it is difficult to form the TFT having an active region made of a polysilicon film over this substrate.
In view of the above problems, it is an object of the present invention to manufacture a semiconductor device including a semiconductor element having an active region made of a polysilicon film and a semiconductor element having an active region made of an amorphous silicon film, typically, a semiconductor device including an optical sensor, a photoelectric conversion element, or a solar battery element over a lightweight and thin, preferably, flexible substrate or organic member.